Both magnetic and optical disk recorders employ either a 14 large plurality of concentric record tracks or a single spiraling record track. The tracks on optical disks are identified by any one of a plurality of surface configurations on the optical medium (disk). A common configuration is a disk-shaped medium having circularly concentric Groove or a single spiral groove, for indicating the location of the record tracks. In both magnetic and optical disks recorders, means are provided for faithfully following one of the addressed record tracks. When it is desired to scan a track other than a currently scanned track, a track seek operation is provided; that is, the track following is aborted and a track seek algorithm is initiated which causes the effective disconnection of the track following operation.
When seeking from one track to another track, magnetic and optical recorder must keep track of its current track on the disk as well as the desired track. This is typically done using a tracking error signal (TES). Tracks are counted as a recording head is moved radially across the tracks between the current track and the desired track. As is well known in the art, during a seek operation, a light beam is moved along with the recording head radially across the tracks. As the light beam moves across the tracks, the TES appears as a sinusoidal or substantial sinusoidal waveform. The peaks of TES are produced at the cliffs between the tracks and grooves on the disk surface. In this manner, the tracks are one full cycle apart.
The simplest method of determining position from the TES is to produce a pulse whenever the TES crosses a zero reference point and then count these pulses. This method, however, is susceptible to miscounting due to noise and disk defects. A superior method, which is used today, utilizes a plurality of TES detectors to indicate zero crossing points and positive and negative peaks. Two peaks of opposite polarities must be encountered before a track crossing is counted. This method allows for greater noise tolerance on the TES. However, media defects can still occasionally result in either the seek operation being too long, i.e. it missed counting a track, or the seek being too short, i.e. counting a defect as a track. As is known in the art, both of these conditions can be recovered from quickly and easily.
The only time a defect can cause a serious problem is when it occurs at the end of the seek operation when a tracking servo is activated. The tracking servo attempts to maintain the TES at the zero reference point. If the tracking servo is activated on a upward slope of the TES when the seeking operation is in a direction toward an outer edge of the disk, the tracking servo will lock on the target track. If, however, the tracking servo is activated on the downward slope, the TES would be opposite to what is needed for a stable condition. Thus, the recording head will "skate" over the disk. In essence, the head will move at an accelerated pace during the downward slope and decelerate over the upward slope. As a result of this activity, a time consuming recovery procedure must activated to stop the skating motion and to lock on the target track. Thus, it is important to activate the tracking servo on the correct slope after a seek operation.
U.S. Pat. No. 4,839,876 discloses an optical disk recorder which includes a position servo control loop for causing a beam of radiation to faithfully follow a track on the disk, jump from a current track to an adjacent track or to a small number of tracks away from the current track. Operation of the position servo loop is altered by selectively reversing the phase of a differentiated position error signal and integrating the differentiated position error signal for comparison with a sawtooth signal for moving the beam of radiation from the current track to an immediately adjacent track under continuous position servo control. Upon reaching the adjacent track, the sawtooth signal returns to a reference potential for initiating track following. The servo error signal and the sawtooth signal does not cause any significant perturbations in the positioning servo loop operation. This patent does not address the problem encountering media defects during a seek operation.
U.S. Pat. No. 5,038,333 discloses a track-seeking apparatus of a disk recorder which employs a track-crossing sensor to produce track-crossing signals. An oscillator is slaved to the sensor for supplying substitute track-crossing pulses in the absence of the sensor providing such pulses or when the radial velocity exceeds a threshold velocity. A velocity profile means alters the oscillator frequency so that the oscillator produces track-crossing pulses in accordance with the profile. This patent does not address the problem of the encountering media defects during a seek operation.
U.S. Pat. No. 5,001,732 discloses a track counter for optical disk which counts the number of tracks a light beam passes upon when an optical head moves over an optical disk in a radial direction. The track counter includes a passage signal generating circuit which generates a signal each time the reflected light of the the light beam passes over a track. A counter counts the signals generated by the passage signal generating circuit. A counter suspending circuit determines that the light beam will pass over a specific position of a track and suspends the action of the counter for a predetermined period of time. A count value compensating circuit assumes the number of tracks the light beam passed during the predetermined period of time and adds this assumed number to the count value of the counter. This invention address problems on the disk in designated areas. It does not address the problem of encountering media defects which may occur in areas other than the designated areas.
Accordingly, it is desired to provide for a drive which is less susceptible to poor optical media quality and provides for more reliability during seek operations.